Automation of the pelleting process wherein dry feed material is mixed with moisture and heated before entering the pellet mill and being pelleted has been substantially advanced through the efforts of the inventor herein who is also an inventor of prior U.S. Pat. Nos. 3,932,736; 4,340,937; and 4,463,430. These prior patents disclose and claim controls which automate the pelleting process by controlling the flow rate of the dry material, the amount of moisture added, and the amount of heat added to the material before it enters the pellet mill. As is described more completely in these prior patents, it is very important that the moisture content and temperature of the material, and the difference in temperature taken at various points in the pellet mill be controlled to assure optimum pelleting for different kinds of materials. Still another prior art patent which addresses this problem is U.S. Pat. No. 3,573,924.
In the prior art, and as disclosed in these prior patents, the material is loaded into a surge hopper, and then fed at a controlled rate through a mixer/conditioner in which the moisture and heat are added to the dry material. The output of the mixer/ conditioner is then input to the pellet mill itself where it is squeezed through a die by a roller, and a plurality of knives cut the pellets to size. Typically, heat can be added to the dry material in a number of ways. Chief among these is by the direct addition of steam to the dry material in the mixer/ conditioner. The addition of steam is effective in raising the temperature of the dry material, but it also adds moisture. For some dry materials this direct addition of steam is very effective. However, for some dry materials it is desired to only add heat as the moisture content of the material may be already great enough to satisfy the pelleting requirement. For these applications, dry heat is typically added by providing a steam jacket which surrounds the mixer/conditioner and through which steam is circulated. Heat is thus added by radiation and direct contact with the walls of the mixer/conditioner.
However, the steam jacketed mixer/conditioner of the prior art does have several drawbacks. First of all, it is quite expensive as it must be sealed and requires a boiler certification. By its very nature, it is also maintenance intensive. From an operational consideration, the surface area which can be utilized to exchange the heat from the steam to the dry material is fixed, and also the temperature of the steam is fixed so that the capacity to deliver heat to the dry material is fixed, and remains constant irrespective of the flow rate of the material through the mixer/conditioner. Thus, as the flow rate of the material varies, so does the amount of heat added by the steam jacket. This results in an indeterminate amount of heat being added to the dry material thereby rendering the control of the addition of dry heat very difficult. Also, the limited throughput of the steam jacket makes it marginally effective at high flow rates of dry material.
To solve these and other problems, the inventor herein has succeeded in developing a way of adding dry heat to the material with microwave energy as it flows through the mixer/conditioner. This can be achieved by utilizing a plurality of magnetrons and placing them at any point in the process upstream of the pellet mill. This can be either in the mixer/ conditioner or in the surge hopper which stores the dry material prior to its entering the mixer/conditioner. To accommodate varying flow rates, groups of magnetrons may be provided, and these groups may be separately enabled as the flow rate of dry material through the pellet mill increases. For example, twenty magnetrons may be installed, and these may be grouped in four groups of five each. Thus, at a low speed, only a single group may be made available for energization and of the five in that single group, anywhere from one to five of these magnetrons may be energized to provide further fine tuning of the heat applied to the dry material. Above a threshold, another group of magnetrons may be enabled for control, and the process extended as the throughput of the mixer/conditioner increases until all four groups of magnetrons are enabled and available for energization. A temperature probe installed directly downstream of the point where the energy is added to the dry feed senses the temperature of the dry feed and is used by the control, in addition to the feeder speed, to determine the number of magnetrons and groups of magnetrons to be energized. A temperature probe immediately above the pellet mill itself may be used to sense the temperature of the material immediately before it enters the mill for pelleting.
Because of the nature of microwave energy, several modifications must be made to accommodate its use in the pellet mill system. To eliminate arcing, the picks in the mixer/conditioner used to move the dry material therethrough must be made of a ceramic substance or a suitable substitute that is non-conductive. Alternately, some other modification must be made to eliminate the problem of arcing in the mixer/conditioner. To satisfy the relevant safety requirements for use of microwave devices, door interlock switches, metal gaskets, and microwave warning devices may also have to be added to the surge hopper or mixer/conditioner.
While the foregoing provides a brief synopsis of the major advantages and features of the present invention, a more complete understanding may be gained by referring to the drawing and description of the preferred embodiment which follows.